1. Field of the Invention
This invention relates to an optical apparatus having means for controlling the position shifting movement of a focusing lens.
2. Description of the Related Art
Many focusing lens driving devices for accurately bringing the position of the focusing lens of an automatic focusing camera to a stop within an in-focus range by driving the lens at a low speed when an out-of-focus degree of an image on a focal plane (hereinafter referred to as a defocus degree) is small and, in the event of a great defocus degree, by driving the lens first at a high speed until the lens position comes to a point which is at a given short distance from an in-focus point before the speed is switched to the low driving speed, have been disclosed, for example, in Japanese Laid-Open Patent Application Nos. SHO 56-94334, SHO 58-18611 and SHO 59-26709, U.S. Pat. No. 4,894,676, etc.
Roughly stated, the procedures taken according to the automatic focus adjusting method employed by these known devices are as follows:
1) The defocus degree DEF is detected by a focus detecting device.
2) The defocus degree DEF detected is converted into a focusing lens driving degree DL by using a degree of sensitivity S which is an amount of movement of the position of an image forming plane relative to a predetermined driving degree of the focusing lens. The conversion can be accomplished according to the following formula: EQU DL=DEF/S
In the case of a single lens which is arranged to be drawn out as a whole, the degree of sensitivity S can be considered to be nearly equal to "1". However, the degree of sensitivity S varies according to zooming, i.e., with the focal length in the case of a zoom lens and also varies with the focusing in the case of an inner focus type lens.
3) The focusing lens driving degree DL is converted into the rotating degree of an actuator used for driving the focusing lens. In this instance, the rotating degree is expressed in general by the number of pulses FP generated by an encoder which is arranged to monitor the rotating degree of the actuator. It is, therefore, expressed as follows: EQU FP=DL / PTH
wherein "PTH" represents the coefficient of "focusing lens driving degree vs. the number of pulses generated" determined by the lead of the helicoid screw of the focusing lens and the gear ratio of the gear train of a driving power transmission system.
4) The focus actuator is driven in a accelerating-and-decelerating pattern in accordance with the value of the pulse number FP outputted. The focusing lens is thus driven to reach an intended in-focus position.
5) The focus detection mentioned in Para. 1) above is again performed. If the defocus degree is found to be within a given in-focus range. The lens is regarded as in focus and the sequence of focusing steps are brought to an end. If not, the sequence of steps of Paragraphs 2) to 5) are continued.
In accordance with the above-stated method, the focusing lens driving degree DL is obtained from the defocus degree DEF by using the degree of sensitivity S. However, in driving the focusing lens, the driving pattern is irrelative to the degree of sensitivity S and is determined only by the focusing lens driving degree DL or the pulse number FP. This is further described as follows with a lens of 35-105 mm/F4 and PTH=0.01 taken up by way of example:
In the case of a zoom lens in general, the degree of sensitivity S varies in proportion to the square of the focal length f of the lens. Assuming that the degree of sensitivity S is at 0.5 when the focal length f is 35 mm, the degree of sensitivity S at f=105 mm is 4.5, which is a very high degree of sensitivity. The in-focus range (or width) is determined by the F number of the lens. However, in the case of the lens taken up by way of example, the F number is assumed to be unvarying over the whole area of the lens. Therefore, the in-focus width remains unchanged irrespectively of the focusing distance of the lens. In this case, the in-focus width is assumed to be plus or minus 0.1 mm (on the focal plane). Then, the value of the ratio between "the in-focus width and the focus shifting degree per pulse" becomes the smallest at the telephoto end position of the lens (f=105 mm). Therefore, the resolving power of the encoder, the gear ratio of the gear train, the focusing lens driving speed, i.e., the accelerating-and-decelerating pattern, etc., are arranged to give a prescribed lens stopping accuracy at the telephoto end of the lens.
With the lens arranged in this manner, a case where the defocus degree DEF is 9 mm at f=105 mm is compared with another case where the defocus degree DEF is 1 mm at f=35 mm as follows: In this instance, the focusing lens driving degree DL is 2 mm in both cases. Then, the number of driving pulses FP is 200 also in both cases. Therefore, in both cases, the lens is driven in the same accelerating-and-decelerating pattern. The driving time T and the stopping accuracy .delta.FP are also the same in both cases. Let us here assume that the driving time T is 0.2 sec and the stopping accuracy .delta.FP is plus or minus 1 pulse. Then, the stopping accuracy at f=105 mm on the focal plane is pulse or minus 45 .mu.m, which is adequate for the in-focus width of plus or minus 0.1 mm, while the stopping accuracy at f=35 mm is plus or minus 5.0 .mu.m, which can be considered to be a somewhat excessive accuracy. The accelerating-and-decelerating pattern obtained in this instance is shown in FIGS. 9 and 10 of the accompanying drawings.
In FIG. 9, the axis of abscissa shows the number of driving pulses and the axis of ordinate the driving speed. Accelerating-and-decelerating curves A1 to A5 show values obtained when the required driving degree FP is at a value FP1. The driving speed is at first accelerated to reach a maximum speed .omega.max when the lens is driven to the point of a pulse number FPa. The speed comes to be decelerated at a point preceding a stopping target point by a number of pulses FPb. After that, the lens is driven at a low constant speed .omega.low. The brake is applied at a point preceding the stopping target point by a number of pulses FPc. Then, the lens position comes to the target point after overrunning to a given degree. A curve shown by a one-dot chain line represents a lens of a heavy driving load. A curve shown by a two-dot chain line represents a lens of a light driving load. The fluctuations in the stopping accuracy due to the weight of the load become .+-..delta.FP.
Curves B1, B3, B4 and B5 are obtained in a case where a required driving degree FP2 is smaller than the required driving degree FP1. In this case, the driving speed is decelerated before the speed reaches the maximum speed .omega.max. Then, the lens is driven at the low speed .omega.low and, after that, the brake is applied to stop the lens. The stopping accuracy thus obtained is also .+-..delta.FP.
FIG. 10 shows the driving speed in relation to time. The axis of abscissa of FIG. 10 shows time from the commencement of lens driving. The axis of ordinate shows the lens driving speed. Curves C1 to C5 correspond to the curves A1 to A5 of FIG. 9, while curves D1 and D3 to D5 correspond to the curves B1 and B3 to B5 of FIG. 9.
During a period up to a point of time t0, the back-lash of the gear train is removed. During this period, a very small current is applied to the focusing lens driving motor for the purpose of canceling the backlash of the gear train between the motor and the helicoid screw. Therefore, several pulses might be received at a pulse encoder during this period. However, this causes no movement of the focusing lens. The pulses received during this period are, therefore, not counted. After that, the lens driving speed is accelerated to the maximum speed .omega.max and decelerated to the constant speed .omega.low before the brake is applied to stop the lens.
A feature of the method lies in the following point: While the area of the constant driving speed is not very long in terms of the driving pulses as viewed on the curve A4 in FIG. 9, it becomes much longer in terms of the time base as viewed on the curve C4 in FIG. 10. In other words, the constant speed driving time occupies a relatively large portion of the whole driving time. A driving time Td, therefore, does not much differ from a driving time Tc, as shown in FIG. 10, even if the number of driving pulses FP2 is one half of the number of driving pulses FP1 shown in FIG. 9.
As described in the foregoing, the accelerating-and-decelerating pattern is set in such a way as to have the driving speed appositely balanced with the lens stopping accuracy at the telephoto end (f=105 mm). Therefore, in a case where the lens is to be used at the wideangle end (f=35 mm), the stopping accuracy is higher than a necessary degree. On the other hand, in a case where the number of driving pulses is 200, for example, it corresponds only to a defocus degree of 2 mm at the wide-angle end on the focal plane while it corresponds to a defocus degree of 9 mm at the telephoto end. Therefore, if the lens is arranged to be driven for an unvarying length of time of 0.2 sec, the speed of a focus adjusting action on the image of a photographing object, as viewed within a view finder, would appear to be extremely slow at the wide-angle end.
When the lens is used on the side of the wide-angle end where the degree of sensitivity S is low, therefore, the operability of the automatic focusing device can be improved by varying the accelerating-and-decelerating pattern of the lens driving speed in such a way as to ease the stopping accuracy and to shorten the driving time.
To solve the above-stated problem, Japanese Laid-Open Patent Application No. SHO 58-194005 disclosed an arrangement whereby the shifting speed of the focused state of an image obtained on the focal plane can be made constant irrespectively of the focal length of a zoom lens by varying a focusing lens driving speed according to information on zooming. According to this arrangement, the lens driving speed is lowered at a fixed rate on the side of the telephoto end from the speed used on the side of the wide-angle end. Therefore, while it effectively renders the image shifting speed, uniform this effect is achieved at the expense of the focusing time required when a great defocus state occurs on the telephoto side. In other words, while it is possible to drive the lens at the maximum speed of the motor covering a fairly large part of the first half portion of the driving area for shortening the driving time when the lens must be driven to a great extent on the telephoto side, it is impossible to do so in accordance with the arrangement disclosed.
Another drawback of the arrangement lies in the following point: In a case where the zoom lens is of the so-called rear focusing type, the degree of sensitivity S varies not only with the focal length of the lens but also with the distance to the object to be photographed, i.e., with the position of the focusing lens of the zoom lens. However, in accordance with the arrangement, it is impossible to make any correction in this connection.
Meanwhile, Japanese Laid-Open Patent Application No. SHO 56-162728 disclosed a focusing lens speed adjusting method for a rear-focusing type zoom lens. In accordance with this method, the lens driving speed is changed according to the focus adjusting distance. An example of embodiment of this method is arranged to lower the lens driving speed on the wide-angle side which has a shorter focus adjusting distance However, as mentioned in the foregoing, the degree of sensitivity S varies approximately in proportion to the square of the zooming ratio of the zoom lens. Therefore, the requirement for stopping accuracy in terms of the rotation angle of the focusing actuator should be eased to a great degree on the wide-angle side. In view of this, the speed adjusting method disclosed cannot be considered to be always appropriate.
Further, in accordance with this method, the maximum speed is also lowered in lowering the lens driving speed. This, therefore, presents the same problem as in the case of the Japanese Laid-Open Patent Application No. SHO 58-194005 mentioned in the foregoing.
Japanese Laid-Open Patent Application No. SHO 62-215217 disclosed a method of varying the focusing lens driving speed according to the degree of sensitivity S in the event of a low contrast search. That method is, however, intended to prevent an in-focus point from being lost to sight in searching for the in-focus point by driving the lens at a lower speed when the defocus degree for the object is located outside the focus detecting range of the camera. Hence, the speed of the constant speed driving area immediately before stopping the lens is not changed. Referring to FIG. 11, this is further discussed as follows: In a case where the degree of sensitivity S is low, a focus detecting action is repeated by driving the lens at the maximum speed .omega.max for the search as indicated by parts E1 to E5 in FIG. 11. Upon detection of the in-focus point, the lens driving speed is slowed down at the part E3. The lens is then driven at the constant driving speed at the part E4. After that, the brake is applied to bring the lens to a stop. In the case of a high sensitivity, the search driving is performed at a speed .omega.2; and, when the in-focus point is detected, the lens is driven at the constant low speed .omega.low before bringing the lens to a stop. In both cases, the lens is driven at the speed .omega.low when the brake is applied. Therefore, the stopping accuracy is plus or minus .delta.FP in both cases. However, on the focal plane, the stopping accuracy of the former (in the case of the low sensitivity) is better than that of the latter to a degree more than necessary.
The examples of the prior art described above are thus considered to be incapable of performing optimum control over the whole lens driving range, because the balance between the driving time and the stopping accuracy is lost when the degree of sensitivity is changed by zooming, etc.